FROM NPR NEWS -- Mitt Romney clinched the Republican presidential nomination Tuesday evening with a win in the Texas primary, a triumph of endurance for a candidate who fought hard to win over skeptical conservative voters he must now fire up for the campaign against President Obama.

According to the Associated Press count, Romney surpassed the 1,144 delegates needed to win the nomination by winning at least 88 delegates in the Texas primary.

Romney, who came up short in the Republican presidential race four years ago, outlasted a carousel of Republican rivals who dropped out of the state-by-state primary contest. None of his former rivals actively campaigned in Texas.

The former Massachusetts governor has reached the nomination milestone with a steady message of concern about the U.S. economy, a campaign organization that dwarfed those of his Republican foes and a fundraising operation second only to that of Obama, his Democratic opponent in the general election.

"I am honored that Americans across the country have given their support to my candidacy and I am humbled to have won enough delegates to become the Republican Party's 2012 presidential nominee," Romney said in a statement.

"Our party has come together with the goal of putting the failures of the last three and a half years behind us," Romney said. "I have no illusions about the difficulties of the task before us. But whatever challenges lie ahead, we will settle for nothing less than getting America back on the path to full employment and prosperity."

In Obama, he will face a well-funded candidate with a proven campaign team in an election that will be heavily influenced by the economy.

One issue that has drawn backing from some of the most conservative Republicans the question of Obama's citizenship resurfaced again on Tuesday.

Romney spent Tuesday evening at a Las Vegas fundraiser with celebrity real estate mogul Donald Trump, who has been renewing discredited suggestions that Obama wasn't born in the United States. Romney says he believes Obama was born in the U.S. but has yet to condemn Trump's repeated insinuations to the contrary.

The Obama campaign released a video Tuesday criticizing Romney's unwillingness to stand up to Trump and the more extreme elements in his party.

"If Mitt Romney lacks the backbone to stand up to a charlatan like Donald Trump because he's so concerned about lining his campaign's pockets, what does that say about the kind of president he would be?" Obama's deputy campaign manager, Stephanie Cutter, said in a statement.

Asked Monday about Trump's contentions, Romney said: "I don't agree with all the people who support me. And my guess is they don't all agree with everything I believe in." He added: "But I need to get 50.1 percent or more. And I'm appreciative to have the help of a lot of good people."

Trump told CNN in an interview Tuesday that he and Romney talk about other issues jobs, China, oil and more and not about the place of Obama's birth or the validity of his birth certificate. Asked how he viewed Romney's position that the president was indeed born in the U.S., Trump said: "He's entitled to his opinion, and I think that's wonderful. I don't happen to share that opinion and that's wonderful also."

Republicans won't officially nominate Romney until late August at the Republican National Convention in Tampa, Florida.

He won at least 88 delegates in Texas with 64 left to be decided, according to early returns. The 152 delegates in Texas are awarded in proportion to the statewide vote. Romney now has 1,174 delegates.

Texas Republicans also voted in a Senate primary to choose a candidate to run for the seat being vacated by Sen. Kay Bailey Hutchison. The party establishment choice, Lt. Gov. David Dewhurst, led the insurgent candidate, state Solicitor General Ted Cruz, and Dallas Mayor Tom Leppert in early returns.

If no one gets more than 50 percent of the vote, the top two finishers will go to a runoff in July. The nominee will be strongly favored to win in November in heavily Republican Texas.

Romney, 65, is clinching the presidential nomination later in the calendar than any recent Republican candidate but not quite as late as Obama in 2008. Obama clinched the Democratic nomination on June 3, 2008, at the end of an epic primary battle with Hillary Rodham Clinton. Four years ago, John McCain reached the threshold on March 4, after Romney had dropped out of the race about a month earlier.

Several other Republican contenders including Newt Gingrich, a former speaker of the U.S. House of Representatives, and Rick Santorum, former U.S. Senator from Pennsylvania earlier dropped out of the race as Romney's well-financed campaign gained momentum.

Romney has been in general-election mode for weeks, raising money and focusing on Obama, largely ignoring the primaries since his competitors dropped out or stopped campaigning.

Libertarian-leaning Texas Rep. Ron Paul said on May 14th he would no longer compete in primaries, though his supporters are still working to gain national delegates at state conventions.

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